Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with high-energy ions. In semiconductor fabrication, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for the IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. A specification of the ion species, doses and energies is referred to as an ion implantation recipe.
FIG. 1 depicts a prior art ion implanter system 100. As is typical for most ion implanter systems, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards a target wafer 120 (located in a wafer plane. 12). A number of measurement devices, such as a dose control Faraday cup 118, a traveling Faraday cup 124, and a setup Faraday cup 122, may be used to monitor and control the ion beam conditions.
For a uniform distribution of dopant ions, an ion beam is typically scanned across the surface of the target wafer. FIG. 2 shows a typical setup for continuous wafer implantation with an ion beam. In an ion implanter system, a wafer 204 may flow slowly along an axis 212 through a wafer chamber. At the same time, an ion beam spot 202 may be scanned horizontally across the surface of the wafer 204. The scanned ion beam spot 202 may form a beam path 20 that has end points 208 and 210. A dose control Faraday cup 206 may be used to measure the ion beam current.
In production, it is desirable to achieve a uniform ion beam profile along the beam path. The process of tuning the ion implanter system to achieve the uniform ion beam profile is called “uniformity tuning.”
FIG. 3 illustrates a prior art method for uniformity tuning. In a setup similar to the one shown in FIG. 2, an ion beam spot 302 is scanned at a constant scan velocity along a path 30 between end points 308 and 310. The ion beam spot 302 is relatively small compared to the size of a wafer 304, which is typical for medium- and high-energy ion beams. In existing uniformity tuning methods for these types of ion beams, it is usually assumed that: (1) the ion beam spot will maintain substantially the same size and deliver substantially the same current during scanning; and (2) the ion beam spot is scanned fully off the wafer edge. It follows from these assumptions that, during scanning, the ion beam spot will only exhibit small changes and any resulting non-uniformity in the ion beam current density distribution of the scanned beam along the beam path is also small. Thus, in existing uniformity tuning methods, the ion beam spot profile is only measured once (unscanned) at the center of the wafer. In FIG. 3, a waveform 32 illustrates an individual beam dose density profile for the ion beam spot 302. As the ion beam spot 302 is swept along the path 30, the ion beam current density of the scanned ion beam at each point is an accumulated effect of one or more individual beam dose density profiles of the ion beam spot 302. A waveform 34 illustrates an ion beam current density distribution of the scanned ion beam between the end points 308 and 310. Since the individual beam dose density profile of the ion beam spot 302 is practically the same everywhere along the path 30, there is a linear relationship between the current density of the scanned beam and the scan velocity. That is, for each location in the path 30, a small increase in the scan velocity will cause a proportional decrease in the ion beam current density at that location, and a small decrease in the scan velocity will cause a proportional increase in the ion beam current density. Existing uniformity tuning methods are based on this assumption of linearity—if the ion beam current density profile 34 is not uniform enough, a typical approach is to adjust, for each unit distance along the path 30, the ion beam spot scan velocity proportionally to the accumulated ion beam current within that unit distance.
It should be noted that the scanned ion beam current density rolls off quickly near both end points 308 and 310. However, since the ion beam spot 302 is scanned fully off the wafer edges, the roll-offs do not affect ion beam coverage of the wafer 304. It should also be noted that, once the ion beam spot 302 has been swept fully off the wafer 304, the ion beam current no longer contributes to the wafer implantation. The ratio between the accumulated ion beam current on the wafer and the total ion beam current accumulated during a full scan is referred to as “beam utilization.” Beam utilization indicates what portion of the total ion beam current is actually utilized for wafer implantation.
The above-described methods for uniformity tuning have been applied to medium- and high-energy ion beams with acceptable results. However, as the semiconductor industry is producing devices with smaller and smaller feature sizes, ion beams with lower energies are required for wafer implantation. Low-energy ion beams present some unique challenges that cannot be tackled with existing methods. For example, a low-energy ion beam tends to produce a current that is much lower than desired because it is difficult to transport low-energy ions due to a space-charge effect. Because of the low current, beam utilization becomes an important factor that affects implantation productivity of low-energy ion beams. And a low-energy ion beam usually has a large beam spot, which can cause problems for both beam utilization and uniformity tuning.
FIG. 4 illustrates a typical low-energy ion beam spot 402 being scanned along a path 40 across a wafer 404. As shown, the size of the ion beam spot 402 is comparable to the size of the wafer 404. Thus, the wafer 404 sees a full ion beam spot in only a small portion of the beam path 40. If the ion beam spot 402 is scanned fully off the wafer 404 as in existing methods, the beam utilization will be much lower. Further, the individual beam dose density profiles 42 of the ion beam spot 402 vary significantly along the path 40, and the resulting scanned ion beam current density profile (waveform 44) is highly non-uniform. As noted above, the existing uniformity tuning methods require that changes in the ion beam spot be small and the non-uniformity in the scanned ion beam profile be small. In the case of low-energy ion beams, the above-described assumption of linearity no longer stands. As a result, the existing methods cannot be applied to an ion implanter system that produces low-energy ion beams.
In view of the foregoing, it would be desirable to provide a solution for uniformity tuning in an ion implanter system which overcomes the above-described inadequacies and shortcomings.